1. Field of the Invention
The present invention relates to a network system and a method of managing a maximum transfer rate or unit of communication data in the network system. Especially, in the case where communication data is transferred in a network system using communication protocol for effecting transmission and reception of the communication data in unit of data of a predetermined amount, e.g., in unit of a packet, the invention relates to a network system which can manage and determine statically and dynamically a maximum value of packet length (maximum transfer unit (MTU)) capable of permitting the communication data to be prevented from being divided in end user systems and network transfer devices interconnected together in the network system, that is, in the course of transfer of the communication data, and a method of managing an optimum value of the maximum transfer unit in the network system.
2. Description of the Prior Art
Generally, in constructing a system in which a plurality of networks (for example, networks based on transmission control protocol/internet protocol (TCP/IP)) exist, the networks are mutually connected via transfer devices (for example, IP routers) to permit routing between end user systems interconnected via the plurality of networks. All of the plurality of networks do not have the same specification but in many applications each of them is constructed to have a different specification.
When considering, from the standpoint of for example the transfer size of data, data transfer in this type of network system wherein the plurality of networks are constructed to have different specifications, it is proved that when data permitted to be transmitted via a network is caused to be transmitted via a different network following routing through a transfer device, the size of a packet of the data is larger than the MTU of the different network and the data cannot be transmitted via the different network.
Conventionally, when data is transferred from one network having a large maximum transfer unit to a different network having a small maximum transfer unit in the network system having the plurality of networks interconnected together, a transfer device interposed between the networks having different maximum transfer units as described above must divide the data.
More particularly, the transfer device has the function to divide a packet received from the one network having a large maximum transfer unit into a plurality of packets which are each smaller than the maximum transfer unit of the different network having a small maximum transfer unit and transmit each of the divided data packets to an end user system behaving as destination via the different network.
In such a prior art network system, when data is transferred via a plurality of networks having different maximum transfer units, the transmission data is inconveniently divided in the course of data transfer and relatively large overhead is required for transfer of the data. As a result, the destination consumes much time for reconstruction of the divided packets and in addition, many packets must be transmitted and so reliability of the data transfer per se is degraded.
Concerning this type of network system, a prior art described in, for example, JP-A-63-131635 is known. This prior art pertains to packet length negotiation in a medium access control layer in the same local area network (LAN).
In the aforementioned prior art, an end user system (transmitting side) ready to transmit data does not at all consider packet lengths which are transferable via all the networks present on a midway route preceding a destination end user system (receiving side). Therefore, in the prior art, it is only possible for the transmitting side end user system to suppress the packet length to below a maximum transfer unit of a network to which the end user system is connected directly. Accordingly, in each network transfer device through which data passes during transfer, the transfer device divides a packet which is larger than a maximum transfer unit of each network connected to the transfer device to make divisional packets commensurate with the maximum transfer unit, thus raising problems that time efficiency (throughput) necessary for data transfer is degraded and reliability of the data transfer per se is consequently degraded.